The use of benzoxazines offers a number of advantages as compared to other thermosetting resins including relatively long shelf-life, molecular design flexibility, low cost, high glass transition temperature (Tg), high modulus, relatively low viscosities, good flame retardant properties, low moisture absorption, no by-products released during curing and very low shrinkage upon curing. Furthermore, benzoxazines are capable of being self-cured upon heating; i.e. there is no need for an additional curing agent. This combination of properties means benzoxazines are potentially attractive for use in aerospace applications. In particular they may be useful as the thermosetting matrix in composite materials. However, currently available multifunctional benzoxazines are glassy solids at temperatures below 120° C. making them difficult to process using standard aerospace techniques such as prepregging and resin infusion.
“Prepregging” refers to the process of impregnating unidirectionally aligned reinforcing fibers or woven fabric with a resin matrix to form prepregs in the form of tapes or sheets. These prepregs are then layered onto each other in a particular orientation on a tool to form a laminate. The prepreg lay-up is then subjected to elevated temperature and pressure to cure and consolidate the composite part. The method of pressure application is dependent on the part and configuration, but the use of an autoclave is most common for high-performance structural parts. The prepregs must have a certain amount of tack and drape in order to mold properly. “Tack” is the ability of prepreg plies to stick together, while “drape” is the ability of the prepreg to conform to different contours.
Resin infusion approach differs from that of conventional prepregging in that dry structural reinforcement fibers are placed into a mold cavity or other shaping tool, and a matrix resin is injected or infused into the structural reinforcement fibers. Resin infusion covers processing techniques such as Resin Transfer Molding (RTM), Liquid Resin Infusion (LRI), Resin Infusion under Flexible Tooling (RIFT), Vacuum Assisted Resin Transfer Molding (VARTM), Resin Film Infusion (RFI) and the like. Such conventional techniques require the resins to be of relatively low viscosity and to be thermally stable at processing temperatures.